The number of Yosemite National Park visitors being warned they may have been exposed to a deadly virus has now grown to 22,000.

Eight visitors have become ill after staying in one of the cabins on park property and becoming infected with hantavirus, a potentially deadly virus carried by mice. Three people have died since mid-summer of hantavirus pulmonary syndrome, a form of pneumonia caused by the virus.

The most recent death was in a West Virginian visitor who passed away this week. No details have been given about the latest victim, at the request of the family.

Park officials have already alerted 10,000 visitors to the park’s camping areas that they may have been exposed to the virus while staying in the park’s tent cabins between June and August. But the warning was expanded this week to another 12,000 visitors who may have used the park’s more remote High Sierra Camps.

The expanded warning came after an eighth illness was confirmed in a man who had stayed in three of the park’s camps, including the higher-elevation campsites.

The man says his symptoms were initially mild, but after hearing about the outbreak, he decided to get tested. Lab results confirmed on Thursday he had the disease.

The seven other confirmed patients are all believed to have contracted the virus while staying in one of the insulated “Signature” tent cabins in a low-elevation area of the park. Those cabins were shut down for disinfection and reconstruction after deer mice were found infesting the double walls of the structures.

All of the confirmed patients lived in the U.S., with most from California. The National Park Service said Wednesday that all the remaining patients were either improving or recovering.

But there are concerns there could be more patients.

The U.S. Centers for Disease Control and Prevention have sent warnings to public health agencies and hospitals across the U.S. to be on the lookout for patients with hantavirus pulmonary syndrome (HPS) symptoms.

The park has also been trying to contact visitors who stayed in the Signature tent cabins from mid-June to August to advise them to seek immediate medical attention if they show symptoms.

But since many of the park’s visitors came from outside of the country, warnings are also being issued around the world. This week, the European CDC and the World Health Organization issued global alerts for travellers to any country to avoid exposure to rodents.

Around 20 per cent of mice carry hantavirus, which can spread to humans when they breathe in dust from the urine, droppings or saliva of infected rodents. The types of hantavirus in the U.S. cannot be transmitted from person to another.

Early HPS symptoms generally begin about one to five weeks after exposure and include fatigue, fever, and muscle aches. About half of patients experience headaches, nausea, dizziness or abdominal pain.

The disease progresses rapidly to include coughing, shortness of breath and then severe difficulty breathing.

The infection can’t be cured but early medical attention increases the chance of survival since the symptoms can be managed with oxygen therapy.